Tuberculosis is a prevalent infectious disease caused by members of the Mycobacterium tuberculosis (M. tuberculosis) complex. Most M. tuberculosis complex infections are asymptomatic, or latent. However, around one in ten latent infections eventually progresses to active disease (usually pulmonary tuberculosis) which, if left untreated, is fatal in over 50% individuals.
Efficient laboratory diagnosis of M. tuberculosis complex infection is a key aspect in controlling the spread of tuberculosis. Moreover, rapid and reliable diagnosis allows the correct treatment regimen to be implemented in a timely fashion.
Traditionally, M. tuberculosis complex infection has been diagnosed by demonstrating mycobacteria in body fluids using microscopic examination (using the Acid Fast Bacilli (AFB) stain) or microbiological culture. However, samples must contain a high concentration of mycobacteria (i.e. from 5 to 10000/ml) in order for microscopic examination to be reliable, and culture-based diagnosis is slow.
Newer methods of diagnosing M. tuberculosis complex infection involve detecting an immune response associated with the infection. Like other mycobacteria, M. tuberculosis stimulates CD4+ and CD8+ T-cells, as well other immune cells, to elicit a strong type-1 proinflammatory-like response involving the secretion of cytokines such as interferon (IFN)-gamma and Tumor Necrosis Factor (TNF)-alpha. IFN-gamma release assays (IGRAs) can be used to detect this delayed-type hypersensitivity (DTH) response. IGRAs are based on the principle that T-cells of sensitized (infected) individuals produce IFN-gamma when they re-encounter M. tuberculosis antigens. Commercially available IGRAs for M. tuberculosis include the original QuantiFERON-TB, and its enhanced versions QuantiFERON-TB Gold and QuantiFERON-TB Gold In-Tube assays (Cellestis International, Carnegie, Australia), the enzyme-linked immunospot (ELISPOT) T SPOT-TB assay (Oxford Immunotec, Oxford, United Kingdom), and various veterinary specialties (Bovigam®, Cervigam®, Primagam®, Prionics, Schlieren-Zurich, Switzerland).
A significant advantage of these IGRAs is their increased specificity for M. tuberculosis complex infection. This is achieved by to their use of specific M. tuberculosis antigens that are encoded in region of difference (RD)1, a genomic segment that is absent from the Bacille Calmette-Guérin (BCG) vaccine and most environmental mycobacteria. RIM antigens used in IGRAs include ESAT6 and CFP10. While diagnosis based on the immune response to such antigens is effective, there is an ongoing need to develop new, alternative antigens for use in diagnostic tests. For instance, it is important that the antigens used in diagnostic tests are different to those used in vaccines in order to avoid false positive results being obtained for vaccinated subjects. ESAT6 in particular has potential for inclusion in M. tuberculosis vaccines. Of course, effective new antigens may also be used in vaccines for preventing or treating M. tuberculosis complex infection.